This invention relates to metallic members or articles including those suitable for making strip and, more particularly, to such articles made by powder metallurgy techniques having a unique combination of thermal and electrical properties and especially well suited for making lead frames of the type used in integrated circuits.
A lead frame is a thin metal structure on which an integrated circuit chip is mounted. It serves as means for supporting the chip and for electrically connecting the chip to other components. After the chip has been attached to the lead frame, it is generally sealed in an insulative and protective material such as a glass, ceramic or plastic, with the lead frame extending through the sealing material to provide connection with the chip.
It is desirable that the metal strip material from which the lead frames are produced have a certain unique combination of properties to enable the efficient manufacture of these components, particularly in mass production. The thermal expansion of the lead frame material should be matched to that of the sealing material over the temperature range at which it is sealed. The general range of thermal expansion coefficients desired for the lead frame material is about 4 to 12.times.10.sup.-6 /.degree.C. averaged over the temperature range of about 25.degree. C. to about 400.degree. C., which is a relatively low expansivity for alloys. Furthermore, it is desirable to provide lead frame material with sufficiently low thermal expansivity to allow direct attachment of the silicon chip, which has low expansivity, using eutectic soldering methods wherein the chip and frame are normally heated to about 400.degree. C. and wherein it is important that the chip and frame have approximately the same expansivity. In accordance with the common practice, hereinafter when an expansion coefficient is given at a certain temperature, it is intended to mean the average expansion coefficient over the temperature range of 25.degree. C. to that temperature.
Another important lead frame property in integrated circuit component manufacture is a relatively high thermal conductivity. This is necessary for heat transfer away from the silicon chip during manufacturing steps such as soldering or brazing as well as during operation of the integrated circuits some of which operate at relatively high power levels. Other desired properties include solderability and sufficient stampability and ductility to permit forming and to resist failure from bending fatigue.
Iron-nickel and iron-nickel-cobalt low-expansion alloys with thermal expansion coefficients below 10.times.10.sup.-6 /.degree.C. at 400.degree. C. have been used for lead frame strip because of their low expansivity. One commonly used material is a 42% nickel iron-nickel alloy which combines low thermal expansivity with good solderability, ductility, and stampability. However, these low-expansion alloys leave much to be desired because of their relatively high cost because of the high nickel content and their relatively low thermal conductivity. One method of overcoming the thermal conductivity problem has been to plate the lead frame with silver which can increase the conductivity to the desired level. However, this is an expensive practice which significantly increases the cost of articles made from these already expensive nickel alloys.